change CODE URL from FTP to HTTP

See issue report on the old prefactor github page: https://github.com/lofar-astron/prefactor/issues/297

as well as commit to master: https://github.com/lofar-astron/prefactor/commit/73809818e7cfac171ce91a12024b7ca10552eee2

docs/source/preparation.rst

@@ -25,7 +25,7 @@ typically obtained from the LOFAR `Long-Term Archive`_.
     
     Older versions of **LINC**: All input measurement-sets for one pipeline run need to be in the same directory.
 
-.. _CODE: ftp://ftp.aiub.unibe.ch/CODE/
+.. _CODE: http://ftp.aiub.unibe.ch/CODE/
 .. _TGSS: http://tgssadr.strw.leidenuniv.nl/doku.php
 .. _GSM:  http://172.104.228.177/
 .. _Long-Term Archive: https://lta.lofar.eu